[DomTree] Provide a way to query if DFSInfo is valid

[hiraditya]

Pass like GVNSink modify CFG use DFS numbers for storing basic blocks (See #90995).
Instead of updating DFS numbers after every modification to CFG,
it is more efficient to do lazily i.e., only when DFS numbers are going to be used by the pass.

[llvmbot]

@llvm/pr-subscribers-llvm-support

Author: AdityaK (hiraditya)

Changes

Pass like GVNSink modify CFG use DFS numbers for storing basic blocks (See #90995).
Instead of updating DFS numbers after every modification to CFG,
it is more efficient to do lazily i.e., only when DFS numbers are going to be used by the pass.

---

Full diff: https://github.com/llvm/llvm-project/pull/91251.diff

1 Files Affected:

• (modified) llvm/include/llvm/Support/GenericDomTree.h (+2)

diff --git a/llvm/include/llvm/Support/GenericDomTree.h b/llvm/include/llvm/Support/GenericDomTree.h
index 4fce9d8bfb2b68..91dd58f1b60873 100644
--- a/llvm/include/llvm/Support/GenericDomTree.h
+++ b/llvm/include/llvm/Support/GenericDomTree.h
@@ -328,6 +328,8 @@ class DominatorTreeBase {
   ///
   bool isPostDominator() const { return IsPostDominator; }
 
+  bool isDFSInfoValid() const { return DFSInfoValid; }
+
   /// compare - Return false if the other dominator tree base matches this
   /// dominator tree base. Otherwise return true.
   bool compare(const DominatorTreeBase &Other) const {

[efriedma-quic]

I don't see why you'd need this; updateDFSNumbers short-circuits if the numbers are already up-to-date.

[hiraditya]

I don't see why you'd need this; updateDFSNumbers short-circuits if the numbers are already up-to-date.

Let's say i want to use DFSIn number, how do i know DFSIn number is valid calling updateDFSNumbers?

The use case is:

• in the beginning of pass i call updateDFSNumbers
• i use dfsin numbers for ordering
• modify (or not) CFG to split basic blocks
• i want to use dfs in numbers.

One approach would be to keep a boolean (CFGModified for example) in the pass itself. but having this API will simplify it.

[efriedma-quic]

how do i know DFSIn number is valid calling updateDFSNumbers?

if (!DT.isDFSInfoValid()) DT.updateDFSNumbers() is exactly the same as DT.updateDFSNumbers(). So I can't see why it would matter... unless you need to do some other bookkeeping to discard the invalid DFS numbers?

And as I noted on the other review, calling DT.updateDFSNumbers() after processing every basic block has extremely bad performance characteristics in edge cases.

[hiraditya]

how do i know DFSIn number is valid calling updateDFSNumbers?

if (!DT.isDFSInfoValid()) DT.updateDFSNumbers() is exactly the same as DT.updateDFSNumbers(). So I can't see why it would matter... unless you need to do some other bookkeeping to discard the invalid DFS numbers?

And as I noted on the other review, calling DT.updateDFSNumbers() after processing every basic block has extremely bad performance characteristics in edge cases.

makes sense. i should be able make it work without this API.